1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a method and apparatus for driving a liquid crystal display.
2. Description of the Related Art
In general, liquid crystal display (LCD) devices control light transmittance of individual liquid crystal cells in accordance with a video signal to displaying image. For example, an active matrix LCD device includes thin film transistors formed at each liquid crystal cell for displaying moving images.
As shown in equations 1 and 2, a response time of an LCD device is slow due to inherent physical characteristics of liquid crystals, such as viscosity and elasticity.τr∝γd2/Δε|V2a−V2F|  (1)
wherein, τr represents a rising time when a voltage is supplied to the liquid crystals, Va is a supplied voltage, VF is Freederic transition voltage at which liquid crystal molecules begin to perform an inclined motion, d is a cell gap of the liquid crystal cells, and γ represents a rotational viscosity of the liquid crystal molecules.τf∝γd2/K  (2)wherein, τf represents a falling time at which the liquid crystals returned to an initial position by elastic restoring force after a voltage supplied to the liquid crystals is removed, K is the inherent elastic constant of the liquid crystals, and γ represents a rotational viscosity of the liquid crystal molecules.
Twisted nematic (TN) mode liquid crystals may have different response times due to physical characteristics of the liquid crystal material and a cell gap. For example, the TN mode liquid crystals commonly have a rising time of about 20 to 80 ms and a falling time of about 20 to 30 ms. Since the liquid crystals have a response time longer than one frame interval, i.e., 16.67 ms, in a NTSC system, of a motion picture, a voltage charged within the liquid crystal cell progresses to the next frame prior to arriving at a target voltage. Therefore, the motion blurring phenomenon in which the screen image of the motion picture is blurred out would be caused.
FIG. 1 is a waveform diagram of brightness variation in accordance with data in a liquid crystal display according to the related art. In FIG. 1, since a display brightness BL corresponding to a data VD cannot achieve a desired brightness due to slow response speed when the data VD is changed from one level to another level, an LCD device cannot display desired color and brightness. Accordingly, a motion-blurring phenomenon appears when images are in motion, and display quality deteriorates due to a reduction in contrast ratio. In order to overcome the slow response time, several devices have been developed. For example, U.S. Pat. No. 5,495,265 and PCT International Publication No. WO 99/055678, which are hereby incorporated by reference, have suggested modulating data in accordance with a presence or absence of change in the data by using a look-up table, i.e., high-speed driving method. The high-speed driving method allows the data to be modulated as shown in FIG. 2. For example, U.S. Pat. No. 5,495,265 and PCT International Publication No. WO 99/055678, which are hereby incorporated by reference, have suggested modulating data in accordance with a presence or absence of change in the data by using a look-up table, i.e., high-speed driving method. The high-speed driving method allows the data to be modulated as shown in FIG. 2.
FIG. 2 is a waveform diagram of brightness variation in accordance with data modulation in a high-speed driving method according to the related art. In FIG. 2, a high-speed driving method modulates input data VD and supplies the modulated data MVD to a liquid crystal cell, thereby obtaining a desired brightness MBL. The high-speed driving method increases proportionally according to the term |Va2−VF2| from Equation 1, wherein response time of the liquid crystals reduces rapidly. Accordingly, the LCD device employing such a high-speed driving method compensates for the slow response time of the liquid crystals by modulating the data value in order to alleviate a motion-blurring phenomenon in moving images, thereby displaying images having undesirable color and brightness.
FIG. 3 is a diagram representing an example of the high speed driving method using 8-bit data according to the related art. In FIG. 3, the high-speed driving method detects a variation in most significant bit data through a comparison of most significant bit data MSB of a current frame Fn with most significant bit data MSB of a previous frame Fn−1. If the variation in the most significant bit data MSB is detected, a modulated data corresponding to the variation is selected from a look-up table so that the most significant bit data MSB is modulated. The high-speed driving method modulates only a part of the most significant bits among the input data for reducing the memory capacity when implemented as hardware.
FIG. 4 is a block schematic diagram of a high-speed driving apparatus according to the related art. In FIG. 4, a high-speed driving apparatus includes a frame memory 43 connected to a most significant bit output bus line 42 and a lookup table 44 connected to the most significant bit output bus line 42 and an output terminal of the frame memory 43.
The frame memory 43 stores most significant bit data MSB for one frame period and supplies the stored data to the lookup table 44. Generally, the most significant bit data MSB are high-order 4 bits among 8 bits of the source data RGB.
The lookup table 44 makes a mapping of the most significant bit data of the current frame Fn input from the most significant bit output bus line 42 and the most significant bit data of the previous frame Fn−1 input from the frame memory 43 into a modulation data table, such as Table 1 or Table 2, to select modulated most significant bit data Mdata. Such modulated most significant bit data Mdata are added to a non-modulated least significant bit data LSB from a least significant bit output bus line 41 before output to a liquid crystal display. As shown in Table 1, a lookup table 44 compares the uppermost 4 bits, i.e., 24, 25, 26 and 27, of the previous frame Fn−1 with the uppermost 4 bits, i.e., 24, 25, 26 and 27, of the current frame Fn and selects a modulated data Mdata in accordance with the compared results.
When the most significant bit data are limited to have 4 bits, the lookup table 44 of a high-speed driving method is implemented as shown in the following Tables 1 or 2.
TABLE 101234567891011121314150023456791012131415151515101345678101213141515151520024567810121314151515153001356781011131415151515400134678911121314151515500123578911121314151515600123468910121314151515700123457910111314151515800123456810111214151515900123456791112131415151000123456781012131415151100123456789111314151512001234567891012141515130012334567810111315151400123345678911121415150001233456789111315
TABLE 2016324864809611212814416017619220822424000324864809611214416019220822424024024024016016486480961121281601922082242402402402403200326480961121281601922082242402402402404800164880961121281601762082242402402402406400164864961121281441761922082242402402408000163248801121281441761922082242402402409600163248649612814416019220822424024024011200163248648011214416017620822424024024012800163248648096128160176192224240240240144001632486480961121441761922082242402401600016324864809611212816019220822424024017600163248648096112128144176208224240240192001632486480961121281441601922242402402080016324848648096112128160176208240240224001632484864809611212814417619222424024000016324848648096112128176176208240
In Tables 1 and 2, the leftmost column is for a data voltage VDn−1 of the previous frame Fn−1 while an uppermost row is for a data voltage VDn of the current frame Fn. Table 1 shows lookup table information in which the most significant bits, i.e., 20, 21, 22 and 23, are expressed by the decimal number format. Table 2 shows look-up table information in which weighting values, i.e., 24, 25, 26 and 27 of the most significant 4 bits are applied to 8 bit data. Modulating only most significant bit data MSB of 4 bits reduces the memory capacity of the lookup table 44. However, the method of comparing 4 bits is problematic in that picture quality deteriorates for an uneven variation among grays and skips. For preventing deteriorating picture quality, the width of the modulated data on the lookup table 44 must be broad enough and input source data must be compared by unit of full bits, i.e., 8 bits.
Table 3 illustrates a lookup table, which has a modulated data of 8 bits and compares source data by unit of full bits of 8 bits.
TABLE 3
When the lookup table compares data by unit of full bits of 8 bits and has previously stored modulated data Mdata of 8 bits, the display quality is excellent for uneven variation of gray values, while a memory capacity rapidly increases. For example, if a lookup table compares data by unit of 8 bits and has modulated data Mdata of 8 bits, its memory capacity extends to 65536×8=524,288 bits. Accordingly, the first term 65536 of the left side is a product of 8-bit source data (256×256) in the previous frame Fn−1 and the current frame Fn, respectively. The second term, 8, is the width, 8 bits, of the modulated data on the lookup table 44. In order to implement red, green, and colors RGB, the lookup table needs a memory capacity of as much as 65536×8×3=1,572,864 bits. Accordingly, if the lookup table adopts an 8-bit comparison method for the high-speed driving, a chip size that stores the lookup table increases and manufacturing costs increase in accordance with the increases of the memory capacity.